Syndiotactic vinylidene aromatic polymerization process

ABSTRACT

A process for polymerizing vinylidene aromatic monomers, such as styrene, to produce polymers having a high degree of syndiotacticity using a catalyst composition comprising a Group 4 metal complex and a hydrocarbylsilane or di(hydrocarbyl)silane catalyst adjuvant is disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority from PCT Application Number PCT/US97/06428, filed Apr. 17, 1997, as well as U.S. Provisional Application Ser. No. 60/017,641, filed May 7, 1996.

The present invention relates to a process for polymerizing vinylidene aromatic monomers, such as styrene, to produce polymers having a high degree of syndiotacticity using a catalyst composition comprising a Group 4 metal complex and a catalyst adjuvant. The resulting polymers may be usefully employed in the preparation of solid objects and articles such as a moldings, films, sheets and foamed objects by molding, casting or the like process.

In U.S. Pat. No. 4,680,353 there is disclosed a process for the preparation of polymers of vinylidene aromatic monomers having a stereoregular structure of high syndiotacticity, by the use of Group 4 metal coordination catalysts and an alumoxane cocatalyst.

In U.S. Pat. No. 5,066,741 there are disclosed certain cationic metal compounds formed by reacting a Group 4 metal complex with ammonium or phosphonium salts of Bronsted acids containing a non-coordinating compatible anion or with cationic oxidizers containing a noncoordinating compatible anion. The complexes are usefully employed as catalysts in the polymerization of polymers of vinylidene aromatic monomers having a stereoregular structure of high syndiotacticity.

In U.S. Pat. No. 5,374,696, certain Group 4 metal complexes wherein the metal is in the +3 oxidation state and their use as addition polymerization catalysts are disclosed. For the teachings contained therein.

According to the present invention there is now provided a novel process for preparing polymers of vinylidene aromatic monomers having a high degree of syndiotacticity. The process comprises contacting at least one polymerizabie vinylidene aromatic monomer under polymerization conditions with a catalyst composition comprising:

a) a Group 4 metal complex corresponding to the formula:

    CP.sub.m MX.sub.n X'.sub.p

wherein:

Cp is a single η⁵ -cyclopentadienyl or η⁵ -substituted cyciopentadienyl group, the substituted cyclopentadienyl group being optionally also bonded to M through a substituent X;

M is a metal of Group 4 or the Lanthanide Series of the Periodic Table;

X each occurrence is an inert anionic ligand of up to 20 nonhydrogen atoms and optionally X and Cp are joined together;

X' is an inert, neutral donor ligand;

m and p are independently 0 or 1;

n is an integer greater than or equal to 1; and

the sum of m and n is equal to the oxidation state of the metal;

b) an activating cocatalyst; and

c) a hydrocarbylsilane or di(hydrocarbyl)silane adjuvant corresponding to the formula:

    R.sub.n SiH.sub.4-n

wherein

R is C₁₋₂₀ hydrocarbyl, and

n is 1 or 2.

Compared to polymerization processes utilizing a similar catalyst composition that lacks the aforementioned adjuvant, the present process achieves a significantly improved conversion of monomer and greater efficiency in use, thereby permitting a reduction in the quantity of metal complex employed. In addition the molecular weight of the resulting product is desirably reduced compared to the polymer molecular weight obtained in the absence of such adjuvant. In addition the present adjuvants may be combined with the use of hydrogen to further improve the conversion and reduce the molecular weight of the resulting polymer product.

All reference to the Periodic Table of the Elements herein shall refer to the Periodic Table of the Elements, published and copyrighted by CRC Press, Inc., 1989. Also, any reference to a Group or Series shall be to the Group or Series as reflected in this Periodic Table of the Elements, utilizing the IUPAC system for numbering groups.

As used herein, the term "syndiotactic" refers to polymers having a stereoregular structure of greater than 50 percent syndiotactic of a racemic triad as determined by ¹³ C nuclear magnetic resonance spectroscopy. Such polymers may be usefully employed in the preparation of articles and objects (for example, via compression molding, injection molding or other suitable technique) having an extremely high resistance to deformation due to the effects of temperature.

Preferred adjuvants used herein include methylsilane, dimethylsilane, phenylsilane and diphenylsilane. Most preferred are phenylsilane and diphenylsilane which due to the fact that they are liquids are readily measured and handled in the polymerization process.

With respect to the metal complexes, illustrative but nonlimiting examples of X include hydrocarbyl, silyl, halo, NR₂, PR₂, OR, SR, and BR₂, wherein R is C₁₋₂₀ hydrocarbyl.

Illustrative but nonlimiting examples of X' include ROR, RSR, NR₃, PR₃, and C₂₋₂₀ olefins or diolefins, wherein R is as previously defined. Such donor ligands are able to form shared electron bonds but not a formal covalent bond with the metal.

Suitable polyalkylalumoxane activating cocatalysts (interchangeably referred to as alumoxanes) for use herein especially include methylalumoxane, isopropyl alumoxane, and modified methylalumoxane, that is methylalumoxane modified with triisopropyl aluminum. In addition to the alumoxane it is often desirable to additionally include a C₁₋₄ trialkyl aluminum compound in the polymerization. Examples include triethyl aluminum, tri n-propyl aluminum, tri isopropyl aluminum, tri n-butyl aluminum, triisobutyl aluminum, and mixtures thereof.

Preferred monocyclopentadienyl and substituted monocyclopentadienyl groups for use according to the present invention are more specifically depicted by the formula: ##STR1## wherein

M is titanium;

X independently each occurrence is hydrogen, halide, R, or OR;

R is C₁₋₁₀ hydrocarbyl group;

X' is a C₄₋₄₀ conjugated diene;

n is 1, 2 or 3;

p is 1 when n is 1, and p is 0 when n is 2 or 3;

R' each occurrence is independently selected from the group consisting of hydrogen, halogen, R, NR₂, PR₂ ; OR; SR or BR₂, or one or two pairs of adjacent R' hydrocarbyl groups are joined together forming a fused ring system.

Preferably, the cyclic moiety comprises a cyclopentadienyl- indenyl-, fluorenyl-, tetrahydrofluorenyl-, or octahydrofluorenyl- group or a C₁₆ hydrocarbyl substituted derivative thereof, n is three, p is zero, X is C₁₋₄ alkyl or alkoxide. Most highly preferred metal complexes comprise pentamethylcyclopentadienyltitanium trimethyl, pentamethylcyclopentadienyltitanium tribenzyl, pentamethylcyclopenta-dienyltitanium trimethoxide, octahydrofluorenyltitanium tribenzyl, octahydrofluorenyltitanium trimethyl or octahydrofluorenyltitanium trimethoxide.

In a preferred embodiment, the metal complex is a metal trialkoxide which is combined with a trialkylaluminum or trialkylboron compound either prior to or simultaneously with the activating cocatalyst to form the active catalyst composition. It is believed, without wishing to be bound by such belief that the trialkylaluminum compound or trialkylboron compound causes the in situ transfer of the alkylgroup to the Group 4 metal complex prior to activation thereof.

The complexes are rendered catalytically active by combination with an activating cocatalyst or by use of an activating technique. Suitable activating cocatalysts for use herein include polymeric or oiigomeric alumoxanes, especially methylalumoxane, triisobutyl aluminum modified methylalumoxane, or diisobutylalumoxane; strong Lewis acids, such as C₁₋₃₀ hydrocarbyl substituted Group 13 compounds, especially tri(hydrocarbyl)aluminum- or tri(hydrocarbyl)boron- compounds and halogenated derivatives thereof, having from 1 to 10 carbons in each hydrocarbyt or halogenated hydrocarbyl group, especially tris(pentafluorophenyl)borane; and nonpolymeric, inert, compatible, noncoordinating, ion forming compounds (including the use of such compounds under oxidizing conditions). A suitable activating technique is bulk electrolysis (explained in more detail hereinafter). Combinations of the foregoing activating cocatalysts and techniques may also be employed if desired. The foregoing activating cocatalysts and activating techniques have been previously taught with respect to different metal complexes in the following references: EP-A-277,003, U.S. Pat. Nos. 5,153,157, 5,064,802, EP-A-468,651, EP-A-520,732, and WO93/23412.

Suitably, polymeric or oligomeric alumoxanes, when used, are present in a molar amount compared to metal complex from 10:1 to 1000:1, preferably from 50:1 to 200:1.

Suitable nonpolymeric, inert, compatible, noncoordinating, ion forming compounds useful as cocatalysts in one embodiment of the present invention comprise a cation which is a Bronsted acid capable of donating a proton, and a compatible, noncoordinating, anion, A⁻. Preferred anions are those containing a single coordination complex comprising a charge-bearing metal or metalloid core which anion is capable of balancing the charge of the active catalyst species (the metal cation) which is formed when the two components are combined. Also, said anion can be displaced by olefinic, diolefinic and acetylenically unsaturated compounds or other neutral Lewis bases such as ethers or nitriles. Suitable metals include, but are not limited to, aluminum, gold and platinum. Suitable metalloids include, but are not limited to, boron, phosphorus, and silicon. Compounds containing anions which comprise coordination complexes containing a single metal or metalloid atom are well known and many, particularly such compounds containing a single boron atom in the anion portion, are available commercially.

Preferably such cocatalysts may be represented by the following general formula:

    (L*-H).sup.+.sub.d A.sup.d-

wherein

L* is a neutral Lewis base;

(L*-H)⁺ is a Bronsted acid;

A^(d-) is a noncoordinating, compatible anion having a charge of d-, and

d is an integer from 1 to 3.

More preferably d is one, that is, A^(d-) is A⁻.

Highly preferably, A⁻ corresponds to the formula:

    [BQ.sub.4 ].sup.-

wherein

B is boron in the +3 formal oxidation state; and

Q independently each occurrence is selected from hydride, dialkylamido, halide, alkoxide, aryloxide, hydrocarbyl, halocarbyl, and halosubstituted-hydrocarbyl radicals, said Q having up to 20 carbons with the proviso that in not more than one occurrence is Q halide.

In a more highly preferred embodiment, Q is a fluorinated C₁₋₂₀ hydrocarbyl group, most preferably, a fluorinated aryl group, especially, pentafluorophenyl.

Illustrative, but not limiting, examples of ion forming compounds comprising proton donatable cations which may be used as activating cocatalysts in the preparation of the catalysts of this invention are tri-substituted ammonium salts such as:

trimethylammonium tetraphenylborate,

methyidioctadecylammonium tetraphenylborate,

triethylammonium tetraphenylborate,

tripropylammonium tetraphenylborate,

tri(n-butyl)ammonium tetraphenylborate,

methyltetradecyloctadecylammonium tetraphenylborate,

N,N-dimethylanilinium tetraphenylborate,

N,N-diethylanilinium tetraphenylborate,

N,N-dimethyl(2,4,6-trimethylanilinium) tetraphenylborate,

trimethylammonium tetrakis(pentafluorophenyl)borate,

methyiditetradecylammonium tetrakis(pentafluorophenyl)borate,

methyidioctadecylammonium tetrakis(pentafluorophenyl)borate,

triethylammonium tetrakispentafluorophenyl)borate,

tripropylammonium tetrakis(pentafluorophenyl)borate,

tri(n-butyl)ammonium tetrakis(pentafluorophenyl)borate,

tri(sec-butyl)ammonium tetrakis(pentafluorophenyi)borate,

N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate,

N,N-diethylanilinium tetrakis(pentafluorophenyi)borate,

N,N-dimethyl(2,4,6-trimethylanilinium) tetrakis(pentafluorophenyl)borate,

trimethytammonium tetrakis(2,3,4,6-tetrafluorophenylborate,

triethylammonium tetrakis(2,3,4,6-tetrafluorophenyl)borate,

tripropylammonium tetrakis(2,3,4,6-tetrafluorophenyl)borate,

tri(n-butyl)ammonium tetrakis(2,3,4,6-tetrafluorophenyl)borate,

dimethyl(t-butyl)ammonium tetrakis(2,3,4,6-tetrafluorophenyl)borate,

N,N-dimethylanilinium tetrakis(2,3,4,6-tetrafluorophenyl)borate,

N,N-diethylanilinium tetrakis(2,3,4,6-tetrafluorophenyl)borate, and

N,N-dimethyl-(2,4,6-trimethylanilinium) tetrakis-(2,3,4,6-tetrafluorophenyl)borate.

Dialkyl ammonium salts such as:

dioctadecylammonium tetrakis(pentafluorophenyl)borate,

ditetradecylammonium tetrakis(pentafluorophenyl)borate, and

dicyclohexylammonium tetrakis(pentafluorophenyl)borate.

Tri-substituted phosphonium salts such as:

triphenyiphosphonium tetrakis(pentafluorophenyl)borate,

methyldioctadecylphosphonium tetrakis(pentafluorophenyl)borate, and

tri(2,6-dimethylphenyl)phosphonium tetrakis(pentafluorophenyl)borate.

Preferred are tetrakis(pentafluorophenyl)borate salts of long chain alkyl mono- and disubstituted ammonium complexes, especially C₁₄ -C₂₀ alkyl ammonium complexes, especially methyidi(octadecyl)ammonium tetrakis(pentafluorophenyl)borate and methyidi(tetradecyl)ammonium tetrakis(pentafluorophenyi)borate.

Another suitable ion forming, activating cocatalyst comprises a salt of a cationic oxidizing agent and a noncoordinating, compatible anion represented by the formula:

    (Ox.sup.e+).sub.d (A.sup.d-).sub.e

wherein

Ox^(e+) is a cationic oxidizing agent having charge e+;

e is an integer from 1 to 3; and

A^(d-), and d are as previously defined.

Examples of cationic oxidizing agents include: ferrocenium, hydrocarbyl-substituted ferrocenium, Ag⁺, or Pb⁺². Preferred embodiments of A^(d-) are those anions previously defined with respect to the Bronsted acid containing activating cocatalysts, especially tetrakis(pentafluorophenyl)borate.

Another suitable ion forming, activating cocatalyst comprises a compound which is a salt of a carbenium ion or silylium ion and a noncoordinating, compatible anion represented by the formula:

    ©.sup.+ A.sup.-

wherein

©⁺ is a C₁₋₂₀ carbenium ion or silylium ion; and

A⁻ is as previously defined.

A preferred carbenium ion is the trityl cation, that is triphenylcarbenium. A preferred silylium ion is triphenylsilylium.

The foregoing activating technique and ion forming cocatalysts are also preferably used in combination with a tri(hydrocarbyl)-aluminum compound having from 1 to 10 carbons in each hydrocarbyl group, an oligomeric or polymeric alumoxane compound, a di(hydrocarbyl)(hydrocarbyloxy)aluminum compound having from 1 to 20 carbons in each hydrocarbyl or hydrocarbyloxy group, or a mixture of the foregoing compounds, if desired. These aluminum compounds are usefully employed for their beneficial ability to scavenge impurities such as oxygen, water, and aldehydes from the polymerization mixture. The molar ratio of aluminum compound to metal complex is preferably from 10,000:1 to 1:1, more preferably from 5000:1 to 10:1, most preferably from 200:1 to 25:1.

Preferred aluminum compounds include C₂₋₆ trialkyl aluminum compounds, especially those wherein the alkyl groups are ethyl, propyl, isopropyl, n-butyl, isobutyl, pentyl, neopentyl, or isopentyl, dialkyl(aryloxy)aluminum compounds containing from 1-6 carbons in the alkyl group and from 6 to 18 carbons in the aryl group (especially (3,5-di(t-butyl)-4-methylphenoxy)diisobutylaluminum), methylalumoxane, modified methylalumoxane and diisobutylalumoxane.

An especially preferred activating cocatalyst comprises the combination of a trialkyl aluminum compound having from 1 to 4 carbons in each alkyl group and an ammonium salt of tetrakis(pentafluorophenyi)borate, in a molar ratio from 1:1 to 40:1, or the combination of such a trialkyl aluminum compound with up to 1000 mole percent, preferably from 50 to 200 mole percent (based on M) of an alkylalumoxane, or both of the foregoing combinations.

The activating technique of bulk electrolysis involves the electrochemical oxidation of the metal complex under electrolysis conditions in the presence of a supporting electrolyte comprising a noncoordinating, inert anion. In the technique, solvents, supporting electrolytes and electrolytic potentials for the electrolysis are used such that electrolysis byproducts that would render the metal complex catalytically inactive are not substantially formed during the reaction. More particularly, suitable solvents are materials that are: liquids under the conditions of the electrolysis (generally temperatures from 0 to 100° C.), capable of dissolving the supporting electrolyte, and inert. "Inert solvents" are those that are not reduced or oxidized under the reaction conditions employed for the electrolysis. It is generally possible in view of the desired electrolysis reaction to choose a solvent and a supporting electrolyte that are unaffected by the electrical potential used for the desired electrolysis. Preferred solvents include difluorobenzene (all isomers), DME, and mixtures thereof.

The electrolysis may be conducted in a standard electrolytic cell containing an anode and cathode (also referred to as the working electrode and counter electrode respectively). Suitably materials of construction for the cell are glass, plastic, ceramic and glass coated metal. The electrodes are prepared from inert conductive materials, by which are meant conductive materials that are unaffected by the reaction mixture or reaction conditions. Platinum or palladium are preferred inert conductive materials. Normally, an ion permeable membrane such as a fine glass frit separates the cell into separate compartments, the working electrode compartment and counter electrode compartment. The working electrode is immersed in a reaction medium comprising the metal complex to be activated, solvent, supporting electrolyte, and any other materials desired for moderating the electrolysis or stabilizing the resulting complex. The counter electrode is immersed in a mixture of the solvent and supporting electrolyte. The desired voltage may be determined by theoretical calculations or experimentally by sweeping the cell using a reference electrode such as a silver electrode immersed in the cell electrolyte. The background cell current, the current draw in the absence of the desired electrolysis, is also determined. The electrolysis is completed when the current drops from the desired level to the background level. In this manner, complete conversion of the initial metal complex can be easily detected.

Suitable supporting electrolytes are salts comprising a cation and an inert, compatible, noncoordinating anion, A⁻. Preferred supporting electrolytes are salts corresponding to the formula:

    G.sup.+ A.sup.- ;

wherein

G⁺ is a cation which is nonreactive towards the starting and resulting complex, and

A⁻ is a noncoordinating, compatible anion.

Examples of cations, G⁺, include tetrahydrocarbyl substituted ammonium or phosphonium cations having up to 40 nonhydrogen atoms. A preferred cation is the tetra-n-butylammonium cation.

During activation of the complexes of the present invention by bulk electrolysis the cation of the supporting electrolyte passes to the counter electrode and A⁻ migrates to the working electrode to become the anion of the resulting oxidized product. Either the solvent or the cation of the supporting electrolyte is reduced at the counter electrode in equal molar quantity with the amount of oxidized metal complex formed at the working electrode.

Preferred supporting electrolytes are tetrahydrocarbylammonium salts of tetrakis(perfluoroaryl) borates having from 1 to 10 carbons in each hydrocarbyl group, especially tetra-n-butylammonium tetrakis(pentafluorophenyl) borate.

In general, the active catalyst can be prepared by combining the metal complex and activator in a suitable solvent at a temperature within the range from ⁻ 100° C. to 300° C. The silane adjuvant may be added separately or simultaneously with the remaining components. The catalyst composition may be separately prepared prior to addition of the monomers to be polymerized or prepared in situ by combination of the vanous components in the presence of the monomers to be polymerized. The catalysts' components are sensitive to both moisture and oxygen and should be handled and transferred in an inert atmosphere.

Suitable solvents or diluents for the catalyst preparation and for the polymerization include any of the solvents known in the prior art including, but not necessarily limited to, straight and branched-chain hydrocarbons such as C₆₋₁₂ alkanes (pentane, hexane, heptane, octane and mixtures thereof); C₆₋₁₂ cyclic and alicyclic hydrocarbons such as cyclohexane, cycloheptane, methylcyclohexane, methylcycloheptane and mixtures thereof and C₆₋₁₂ aromatic and alkyl-substituted aromatic compounds such as benzene, toluene, xylene, decalin, and mixtures thereof, as well as mixtures of the foregoing compounds.

The catalysts may be employed as homogeneous catalysts or supported on the surface of a suitable support such as alumina, silica or a polymer.

In the practice of the present invention, suitable vinylidene aromatic monomers include those represented by the formula: ##STR2## wherein each R* is independently hydrogen; an aliphatic, cycloaliphatic or aromatic hydrocarbon group having from 1 to 10, more suitably from 1 to 6, most suitably from 1 to 4, carbon atoms; or a halogen atom. Examples of such monomers include, styrene, chlorostyrene, n-butylstyrene, vinyltoluene, and α-methyistyrene, with styrene being especially suitable. Copolymers of styrene and the above vinylidene aromatic monomers other than styrene can also be prepared.

The polymerization may be conducted under slurry, bulk or suspension polymerization conditions or other suitable reaction conditions including solid, powdered reaction conditions. The polymerization can be conducted at temperatures of from 0° C. to 160° C., preferably from 25° C. to 100° C. for a time sufficient to produce the desired polymer. Typical reaction times are from one minute to 100 hours, preferably from 1 to 10 hours. The optimum reaction time or reactor residence time will vary depending upon the temperature, solvent and other reaction conditions employed. The polymerization can be conducted at subatmospheric pressure as well as super-atmospheric pressure, suitably at a pressure within the range of 1 to 500 psig (6.9 kPa-3,400 kPa). The use of ambient or low pressures, for example, 1-5 psig (6.9-34.5 kPa) is preferred in view of lower capital and equipment costs.

The polymerization may be conducted in the presence of an inert diluent or solvent or in the absence thereof, that is, in the presence of excess monomer. Examples of suitable diluents or solvents include C₆₋₂₀ aliphatic, cycloaliphatic, aromatic and halogenated aliphatic or aromatic hydrocarbons, as well as mixtures thereof. Preferred diluents comprise the C₆₋₁₀ alkanes, toluene and mixtures thereof. A particularly desirable diluent for the polymerization is iso-octane, iso-nonane or blends thereof such as lsopar-ETM, available from Exxon Chemical Company. Suitable amounts of solvent are employed to provide a monomer concentration from 5 percent to 100 percent by weight.

The molar ratio of the vinylidene aromatic monomer to catalyst (in terms of M) may range from 100:1 to 1×10¹⁰ :1, preferably from 1000:1 to 1×10⁶ :1.

As in other similar polymerizations, it is highly desirable that the monomers and solvents employed be of sufficiently high purity that catalyst deactivation does not occur. Any suitable technique for monomer purification such as devolatilization at reduced pressures, contacting with molecular sieves or high surface area alumina, deaeration, or a combination thereof may be employed.

Purification of the resulting polymer to remove entrained catalyst and cocatalyst may also be desired by the practitioner. Such contaminants may generally be identified by residues of ash on pyrolysis of the polymer that are attributable to catalyst or cocatalyst metal values. A suitable technique for removing such compounds is by solvent extraction, for example, extraction utilizing hot, high boiling chlorinated solvents, acids or bases such as caustic followed by filtration.

Having described the invention, the following examples are provided as further illustrative and are not to be construed as limiting. The skilled artisan will appreciate that the invention disclosed herein may be practiced in the absence of any component which has not been specifically disclosed. Unless stated to the contrary, all parts and percentages are based on weight.

EXAMPLES 1-6

All reactions and manipulations were carried out under inert atmosphere in a dry box. Styrene monomer was purified by removing oxygen, passing through activated alumina, and hydrogenation using Pd on alumina to remove phenylacetylene. Solvent was purified by sparging with N₂ and passing through activated alumina and handled using standard inert atmosphere techniques. Catalyst solutions were prepared in volumetric flasks using toluene solvent. The required amount of pentamethylcyclopentadienyltitanium trimethoxide (Cp*Ti(OCH₃)₃) was weighed and added to the flask and toluene then added to form a 0.03 M solution.

A catalyst premix was prepared by combining 1.5 mL of a 1.5 M toluene solution of polymethylalumoxane (PMA), 750 μL of a 1 M toluene solution of triisobutylaluminum (TIBA), 1000 μL of the pentamethylcyclopentadienyltitanium trimethoxide solution, and sufficient toluene to make 10 mL total volume.

Polymerizations were carried out in septum capped, crimp sealed ampoules. The ampoules were charged with 5 ml of styrene. Varying quantities of phenyisilane (1 M in toluene) were added by microliter syringe. Hydrogen was added to some of the ampoules by injecting 1210 μL via a gas tight syringe. The ampoules were equilibrated at 50° C. or 70° C. in a water bath and 42 μL of the catalyst premix was added. After 30 minutes reaction time the polymerization was quenched by the addition of methanol. Each polymer sample was isolated and dried at 150° C. for 30 minutes and then at 250° C. for 30 minutes then weighed in order to determine the percent conversion. Molecular weight of the resulting syndiotactic polymer was determined via standard solution viscometry using atactic polystyrene standards. All polymers had melting points in excess of 260° C. consistent with tacticities of greater than 50 percent based on a racemic triad. Results are shown in Table I.

                  TABLE I                                                          ______________________________________                                              Temp.             Silane:Ti                                                                              conversion                                        Ex. (° C.) H.sub.2 (μL) mole ratio (percent) Mw ×                                                   10.sup.6                                ______________________________________                                         1    50       0        400     46.2    --                                        2 " 0 600 49.3 1.7                                                             3 " 0 800 47.2 --                                                              4 " 1210 550 69.3 1.3                                                          5 70 0 550 31.2 1.1                                                            6 70 1210 550 42.6 0.7                                                         A* 50 0 0 32.7 2.3                                                             B* " 0 0 35.8 --                                                               C* 70 0 0 28.9 1.6                                                           ______________________________________                                          *Comparative                                                             

The results of the above experiments indicate that the use of the adjuvant allows greatly increased conversion of monomer in the reaction times tested. In addition the production of lower molecular weight polymers at the same reaction conditions (compare Ex 2 and Comparative A*, or Example 5 and Comparative C*) show a further beneficial property in the catalyst compositions of the invention. That is, even greater reduction of molecular weight and improved conversion is obtained by the combined use of both hydrogen and the silane.

EXAMPLES 7-9

The reaction, termination and recovery conditions of Examples 1-6 were substantially repeated using reaction times of 60 minutes and the metal complex octahydrofluorenyltitanium trimethoxide (OFTT). A catalyst premix was prepared by combining 1.5 mL of a 10 weight percent toluene solution of polymethylalumoxane (PMA), 750 μL of a 1 M toluene solution of TIBA, 1000 μL of the titanium complex solution, and sufficient toluene to make 10 mL total volume.

Polymerizations were carried out in septum capped, crimp sealed ampoules charged with 10 ml of styrene at 50, 70 and 90° C. The molar ratio of reactants (styrene:MAO:TIBA:OFTT:phenylsilane) was 200,000:75:25:1:370. Conversions at 50, 70 and 90° C. were 88, 91 and 82 weight percent respectively. Repeating the polymerizations at the same temperatures without the use of phenylsilane gave conversions of 69, 54 and 35 weight percent respectively. 

We claim:
 1. A process for preparing syndiotactic polymers of vinylidene aromatic monomers comprising contacting one or more vinylidene aromatic monomers under polymerization conditions with a catalytically effective amount of a catalyst composition comprising:a) a Group 4 metal complex corresponding to the formula: ##STR3## wherein M is titanium; X independently each occurrence is hydrogen, halide R, or OR; R is C₁₋₁₀ hydrocarbyl group; X' is a C₄₋₄₀ conjugated diene; n is 1, 2 or 3; p is 1 when n is 0, and p is 0 when n is 2 or 3; R' each occurrence is independently selected from the group consisting of hydrogen, halogen, R, NR₂, PR₂ ; OR; SR or BR₂, or one or two pairs of adjacent R' hydrocarbyl groups are joined together forming a fused ring system; b) an activating cocatalyst; and c) a hydrocarbylsilane or di(hydrocarbyl)silane adjuvant.
 2. The process according to claim 1 wherein the vinylidene aromatic monomer is represented by the formula: ##STR4## wherein each R* is independently hydrogen; an aliphatic, cycloaliphatic or aromatic hydrocarbon group having from 1 to 10, carbon atoms; or a halogen atom.
 3. The process according to claim 2 wherein the vinylidene aromatic monomer is styrene.
 4. The process according to claim 1 wherein the adjuvant is phenylsilane or diphenylsilane.
 5. The process of claim 1 wherein the catalyst composition comprises:a) a pentamethylcyclopentadienyltitanium tri(C₁₋₄)alkoxide or an octahydrofluorenyltitanium tri(C₁₋₄)alkoxide in combination with a tri(C₁₋₆)alkylaluminum; b) phenyisilane or diphenyisilane; and c) methylalumoxane.
 6. The process of claim 5 wherein the catalyst composition comprises pentamethylcyclopentadienyltitanium trimethoxide. 